"Unsuggester takes 'people who like this also like that' and turns it on its head. It analyzes the thirteen million books LibraryThing members have recorded as owned or read, and comes back with books least likely to share a library with the book you suggest."

I tried it out on a solid fantasy: George R R Martin's A Game of Thrones. Fans of this are least likely to buy The knowledge of the holy : the attributes of God : their meaning in the Christian life, by A. W. Tozer. Strangely, and I don't know what this says about Martin's fans, but the following four "unsuggested" titles are all on Christian theology...